Can you provide a source to back this up?
– Ryan AndersonDec 4 '12 at 14:23

Nope, that is why I said 'should' and 'think' instead of will. I'm just speculating.
– StefanDec 5 '12 at 0:04

@Stefan is correct. Food at temperatures between 40f and 140f is in the 'danger zone' where bacteria grows rapidly. Sous vide will prevent any new bacteria on the food (pouch) and will thaw faster than room temperature (less time in the zone). Thawing in the fridge takes a long time at a place where there is bacteria from other food present and still active (albeit, slow). If your final cooking temperature is safe, you're ok anyway, if it's not you're risking it anyway. Makes this a bit moot.
– MandoMandoDec 11 '12 at 4:11

If we want to minimize the time in the danger zone, then the best strategy is to thaw the frozen bag in the fridge first, then to sous vide it. If one puts a frozen bag in the warm water, then then core will be frozen. The energy used to thaw the core will bring down the temperature around the core. Thus the time spent in the danger zone, will take longer, than if the core was non-frozen.
– soegaardJan 23 '13 at 19:39

@soegaard Yes maybe, but then consider what MandoMando said, think in general it does not make much difference. If you thaw a hole cow it does matter, but for 'small' things that can be thawed in a few hours in a sous vide cooker it should not matter much.
– StefanJan 26 '13 at 6:13

I believe this question was asked in the modernist cuisine forums, which I am having difficulty pulling up at this time, but I recall the answer being something to the tune of "no it is not any major risk because the surface of the meat will reach the bath temperature relatively quickly and the increase in the length of time in the bath will make up for the freezing" but i believe this may be a different story for ground meats since there is so much more surface area that will not be in contact with the bath initially due to the block it will be frozen into. I will update with links when I can access that site again.